Chapter Summary, Questions Answers - Metabolism of Monosaccharides and Disaccharides

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Chapter: Biochemistry : Metabolism of Monosaccharides and Disaccharides

The major source of fructose is sucrose, which, when cleaved, releases equimolar amounts of fructose and glucose.



The major source of fructose is sucrose, which, when cleaved, releases equimolar amounts of fructose and glucose (Figure 12.8). Transport of fructose into cells is insulin independent. Fructose is first phosphorylated to fructose 1-phosphate by fructokinase and then cleaved by aldolase B to dihydroxyacetone phosphate and glyceraldehyde. These enzymes are found in the liver, kidney, and small intestinal mucosa. A deficiency of fructokinase causes a benign condition (essential fructosuria), but a deficiency of aldolase B causes hereditary fructose intolerance (HFI), in which severe hypoglycemia and liver failure lead to death if fructose (and sucrose) in the diet is not eliminated. Mannose, an important component of glycoproteins, is phosphorylated by hexokinase to mannose 6-phosphate, which is reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Glucose can be reduced to sorbitol (glucitol) by aldose reductase in many tissues, including the lens, retina, Schwann cells, liver, kidney, ovaries, and seminal vesicles. In cells of the liver, ovaries, and seminal vesicles, a second enzyme, sorbitol dehydrogenase, can oxidize sorbitol to produce fructose. Hyperglycemia results in the accumulation of sorbitol in those cells lacking sorbitol dehydrogenase. The resulting osmotic events cause cell swelling and may contribute to the cataract formation, peripheral neuropathy, nephropathy, and retinopathy seen in diabetes. The major dietary source of galactose is lactose. The transport of galactose into cells is not insulin dependent. Galactose is first phosphorylated by galactokinase (a deficiency results in cataracts) to galactose 1-phosphate. This compound is converted to uridine diphosphate (UDP)-galactose by galactose 1-phosphate uridyltransferase (GALT), with the nucleotide supplied by UDP-glucose. A deficiency of this enzyme causes classic galactosemia. Galactose 1-phosphate accumulates, and excess galactose is converted to galactitol by aldose reductase. This causes liver damage, severe intellectual disability, and cataracts. Treatment requires removal of galactose (and lactose) from the diet. For UDP-galactose to enter the mainstream of glucose metabolism, it must first be converted to UDP-glucose by UDP-hexose 4-epimerase. This enzyme can also be used to produce UDP-galactose from UDP-glucose when the former is required for the synthesis of structural carbohydrates. Lactose is a disaccharide that consists of galactose and glucose. Milk and other dairy products are the dietary sources of lactose. Lactose is synthesized by lactose synthase from UDP-galactose and glucose in the lactating mammary gland. The enzyme has two subunits, protein A (which is a galactosyltransferase found in most cells where it synthesizes N-acetyllactosamine) and protein B (α-lactalbumin, which is found only in the lactating mammary glands, and whose synthesis is stimulated by the peptide hormone prolactin). When both subunits are present, the transferase produces lactose.

Figure 12.8 Key concept map for metabolism of fructose and galactose. GALT = galactose 1-phosphate uridylyltransferase; UDP = uridine diphosphate; P = phosphate.


Study Questions

Choose the ONE best answer.


12.1 A nursing female with classic galactosemia is on a galactose-free diet. She is able to produce lactose in breast milk because:

A. galactose can be produced from fructose by isomerization.

B. galactose can be produced from a glucose metabolite by epimerization.

C. hexokinase can efficiently phosphorylate galactose to galactose 1-phosphate.

D. the enzyme affected in galactosemia is activated by a hormone produced in the mammary gland.

Correct answer = B. Uridine diphosphate (UDP)-glucose is converted to UDP-galactose by UDP-hexose 4-epimerase, thereby providing the appropriate form of galactose for lactose synthesis. Isomerization of fructose to galactose does not occur in the human body. Galactose is not converted to galactose 1-phosphate by hexokinase. A galactose-free diet provides no galactose. Galactosemia is the result of an enzyme deficiency.


12.2 A 5-month-old boy is brought to his physician because of vomiting, night sweats, and tremors. History revealed that these symptoms began after fruit juices were introduced to his diet as he was being weaned off breast milk. The physical examination was remarkable for hepatomegaly. Tests on the baby’s urine were positive for reducing sugar but negative for glucose. The infant most likely suffers from a deficiency of:

A. aldolase B.

B. fructokinase.

C. galactokinase.

D. β-galactosidase.

Correct answer = A. The symptoms suggest hereditary fructose intolerance, a deficiency in aldolase B. Deficiencies in fructokinase or galactokinase result in relatively benign conditions characterized by elevated levels of fructose or galactose in the blood and urine. Deficiency in β-galactosidase (lactase) results in a decreased ability to degrade lactose (milk sugar). Congenital lactase deficiency is quite rare and would have presented much earlier in this baby (and with different symptoms). Typical lactase deficiency (adult hypolactasia) presents at a later age.


12.3 Lactose synthesis is essential in the production of milk by mammary glands. In lactose synthesis:

A. galactose from galactose 1-phosphate is transferred to glucose by galactosyltransferase (protein A), generating lactose.

B. protein A is used exclusively in the synthesis of lactose.

C. α-lactalbumin (protein B) regulates the specificity of protein A by increasing its Km for glucose.

D. protein B expression is stimulated by prolactin.

Correct answer = D. The expression of α-lactalbumin (protein B) is increased by the hormone prolactin. Uridine diphosphate–galactose is the form used by the galactosyltransferase (protein A). Protein A is also involved in the synthesis of the amino sugar, N-acetyllactosamine. Protein B increases the affinity of protein A for glucose and, so, decreases the Km.


12.4 A 3-month-old girl is developing cataracts. Other than not having a social smile or being able to track objects visually, all other aspects of the girl’s examination are normal. Tests on the baby’s urine are positive for reducing sugar but negative for glucose. Which enzyme is most likely deficient in this girl?

A. Aldolase B

B. Fructokinase

C. Galactokinase

D. Galactose 1-phosphate uridylyltransferase

Correct answer = C. The girl is deficient in galactokinase and is unable to appropriately phosphorylate galactose. Galactose accumulates in the blood (and urine). In the lens of the eye, galactose is reduced by aldose reductase to galactitol, a sugar alcohol, which causes osmotic effects that result in cataract formation. Deficiency of galactose 1-phosphate uridylyltransferase also results in cataracts but is characterized by liver damage and neurologic effects. Fructokinase deficiency is a benign condition. Aldolase B deficiency is severe, with affects on several tissues. Cataracts are not typically seen.

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